A high cholesterol level in serum can be lowered effectively by altering the intestinal metabolism of lipids. In this case the aim may be to hamper the absorption of triglycerides, cholesterol or bile acids. It has been observed in a number of investigations that certain plant sterols, such as β-sitosterol (24-ethyl-5α-cholestane-3β-ol) and its hardened form, β-sitostanol (24-ethyl-5α-cholestane-3β-ol), lower serum cholesterol levels by reducing the absorption of dietary cholesterol from the intestines (1-25). The use of plant sterols can be considered safe, since plant sterols are natural components of vegetable fats and oils. Plant sterols themselves are not absorbed from the intestines, or they are absorbed in very low concentrations. A decreased incidence of coronary disease is clearly associated with a decrease in serum cholesterol, in particular LDL cholesterol. A high serum cholesterol value is the most significant single indicator of the risk of coronary disease.
The degree of cholesterol absorption depends on a hereditary property, apoprotein E-phenotype. Apoprotein E is a protein which belongs to serum lipoproteins and takes part in the transport of cholesterol in the system (26). Of alleles associated with the synthesis of apoprotein E, i.e. the lipoprotein which affects absorption, there are known three types, e2, e3, and e4, which combine in pairs at random. Alleles are capable of forming in total six different combinations. The higher the sum of the subindices, the better absorbable the cholesterol and the higher the level of cholesterol, in particular bad LDL cholesterol, in the serum (27). e4 allele is over represented among the hereditary factors of Finns, so that its proportion is almost double as compared with many European populations (28).
Finns are indeed exceptionally sensitive to dietary flaws and to fatty and high-cholesterol food (29).
Serum cholesterol levels can be lowered by dietary means, by paying attention to the quantity and type of the fat ingested and to the amount of cholesterol intake. In practice, however, these means do not always lead to a satisfactory end result. Other methods, suitable for the entire population, for reaching serum cholesterol levels lower than the present ones must be searched for. Increasing the fiber content of food is a method of limited effect. The cholesterol-lowering effect of soluble fiber in food is based on the binding and removal of bile acids. Since the absorption of cholesterol is of fundamental significance in the regulation of the cholesterol level in serum, it is logical to aim at developing methods by which the absorption of cholesterol can be prevented or reduced.
Pollak demonstrated that ingested plant sterol lowered the level of serum cholesterol in man (1). The same had previously been observed in experimental animals (2, 3). It has been observed in a number of investigations that large doses of plant sterols lower the levels of serum cholesterol, at best by 10-20% (4, 5). In these experiments, large amounts, up to 24 g/day, of β-sitosterol in crystalline form were used (6). In certain experiments the serum cholesterol level was lowered significantly even with lower doses (7), although a small amount of soluble sitosterol administered in the form of fatty acid esters did not seem to lower serum cholesterol very effectively (8). Sitosterol preparations have in general been well tolerated in long-term use (9).
Natural plant sterols resemble cholesterol in their structure. The differences between a cholesterol molecule and a plant sterol molecule are primarily found in the structure of the side chain of the basic frame. An ordinary diet contains plant sterols 100-300 mg/day. Most of the plant sterol in the diet is β-sitosterol, and approximately one-third is campesterol. Small amounts of saturated 5α-sitostanols are also present in food. Usually the campesterol concentrations in serum in particular reflect the degree of absorption of cholesterol (10, 11, 12). Variation in the amounts of plant sterols in the diet affects the serum cholesterol level, but this is an area which has not been studied much. Plant sterols are poorly absorbed from the intestines. Plant sterols which are scantily absorbed into the system (less than 10% of the sterols) (30, 31, 32) are excreted in the bile and through that in the stools. At present it is easy to measure sterol levels from food, serum or stool samples by gas chromatographic methods. The levels in serum are in part dependent on the plant sterol amounts derived from the diet and in part on the efficiency of the absorption of sterols. In general the plant sterol levels in serum remain below 1/300 of the serum cholesterol level, since the absorbed plant sterol fraction is excreted from the system in the bile.
Even large ingested doses of plant sterols do not show in serum plant sterol levels. The values remain at the normal level, since in man the plant sterol absorption capacity is rapidly saturated. The serum plant sterol level rises to a detrimental level in a few rare diseases such as cerebrotendinotic xanthomatosis and sitosterolemia (33, 34, 35), in connection with which coronary disease is common. The incidence of these diseases is at maximum a few cases in a population of one million. Not a single case of these diseases has been observed in Finland. High plant sterol values are at times observed in patients suffering from certain hepatic diseases (36).
Studies of the metabolism of cholesterol have shown that sitosterol inhibits the absorption of both endogenic and dietary cholesterol from the intestines (13, 14). As a result of this, the excretion of neutral steroids in the stools increases, which leads to a shortage of cholesterol in the liver and through that to a decreased serum cholesterol level. On the other hand, sitosterol does not affect the absorption of bile acids (13).
On the basis of experiments on animals, it seems that the action of sitosterol is based on its ability to displace dietary cholesterol in bile acid micelli (15, 16, 17). Similar results have also been obtained in man (37). Various plant sterols have been demonstrated to affect in different ways the absorption of cholesterol (19, 38). Previous studies carried out on experimental animals give the impression that sitostanol is the most effective inhibitor of cholesterol absorption (38) and is itself almost nonabsorbable. Furthermore, an uncontrolled study on six subjects showed that free sitostanol (1.5 g/day) lowered the serum cholesterol (mainly LDL cholesterol) in four weeks by as much as 15%. During a pause of two weeks, the cholesterol values returned to the previous levels (20). Most plant sterol preparations contain a number of different plant sterols. The effect of a plant sterol mixture on the absorption of cholesterol varies, as does their own absorption (21, 22, 23).
The studies carried out so far have mainly concentrated on investigating how the form (crystalline, suspension, granular) in which plant sterols are dosed affects their efficacy in lowering serum cholesterol levels. Crystalline plant sterols do not to a significant degree dissolve in the micelli phase in the alimentary canal, and are therefore not capable of efficiently inhibiting cholesterol absorption. Oils and fats are only to a limited degree capable of dissolving free sterols. Only in a dissolved form do sterols inhibit the absorption of cholesterol. According to Heinemann and coworkers (24), sitostanol inhibited in an infusion experiment the absorption of cholesterol 82%, whereas sitosterol inhibited the absorption 50%.
In certain studies, fatty acid esters of sitosterol, such as sitosterol acetate or oleate or stigmasterol oleate dissolved in fat, have been used. In experiments on rats an “oil” of this type, having a sterol concentration up to 8%, reduced the absorption of cholesterol by 20-40% (22). During a high-cholesterol diet (500 mg/day), sitosterol oleate (2 g/day) dissolved in fat decreased the absorption of cholesterol in the test subjects on average by 33% (25). In the same study, sitosterol mixed with food and in a lower dose (1 g/day) decreased the absorption of cholesterol by 42%.
A German patent (Deutsches Patentamt, Offenlegungsschrift 2035069/Jan. 28, 1971) relates to the adding of plant sterol fatty acid esters to cooking oil with the objective of lowering the serum cholesterol levels in man. The said patent proposes for use in the esterification of free sterols a method which in no case fulfills the requirements for the preparation of a food-grade product. According to the patent, the esterification is carried out between a free sterol and a fatty acid anhydride, with perchloric acid acting as a catalyst. The catalyst and reagent used cannot be accepted in a food process. In addition, the said patent relates to the fatty acid esters of only native plant sterols.
Many reagents which cannot be accepted as a food or for the production of a product intended as an additive for foods have been used in the preparation of sterol fatty acid esters. The use of, for example, chlorine (39), bromine (40), thionyl chloride (41) or anhydride derivatives of fatty acids is common. Of the methods previously patented, only the method of Baltes (Deutsches Patentamt, Offenlegungsschrift 2248921/Apr. 11, 1974) for the esterification of sterols present in oils and fats by a chemical interesterification technique fulfills the criteria of food processes. In the said patent, free sterol and an excess of fatty acid esters are added to a mixture of oil or fat, whereafter the entire fatty mixture is interesterified by a commonly known interesterification technique.